Various working machine control devices have been proposed for the purpose of making the operations of construction machines or the like easier and to reduce burdens on operators. As one of such devices, a hydraulic cylinder has been proposed which is used for each section of a working machine, whose operating stroke is detected, and to which a hydraulic cylinder operating stroke sensor is attached. This hydraulic cylinder includes a type having a built in stroke sensor provided in its central portion to be protected from earth, sand, muddy water and the like.
FIGS. 7 and 8 are cross-sectional views of head portions of cylinders with built-in stroke sensors in accordance with the prior art wherein a sensor accommodation tube 7 is provided in a cavity provided at the bottom of a fluid pressure cylinder and at the center of a piston rod 2, and wherein a stroke sensor using a potentiometer or the like is provided in the sensor accommodation tube 7.
The stroke sensor is composed of a sensor body 5 having a resistor, and a sensor rod 6 having a brush which slides on the surface of the resistor. An end of the sensor body 5 is supported on the unillustrated bottom, and an end of the sensor rod 6 is attached by screwing with nuts 14 and 15 to a plate 24 fitted to the piston rod 2 at an end of the cavity thereof. A rod head 3 is fixed to an end of the piston rod 2.
As fluid pressure is produced for action in the cylinder 1 so that the piston rod 2 is moved reciprocatively, the sensor rod 6 of the stroke sensor is also moved with the movement of the piston rod 2, the brush sliding on the resistor electrically detects a voltage value of the resistor, and this value is supplied to a controller through an electric wiring.
If a rotary motion of the piston rod 2 in a circumferential direction is allowed as well as the axial reciprocating motion when this fluid pressure cylinder is operated, a structure is required in which the stroke sensor is attached while the sensor rod 6 is made concentric with the piston rod 2, and in which the sensor rod 6 is freely rotatable relative to the brush. To reduce the outside diameter of the sensor body 5a, it is necessary to make the the sensor rod 6 eccentric to the sensor body 5a and to limit the direction in which the brush faces to one direction.
In the case of the stroke sensor used as shown in FIG. 7 where the center of the sensor body 5 and the center of the sensor rod 6 coincide with each other, the outside diameter of the sensor body 5 is large in comparison with the stroke sensor shown in FIG. 8 in which the sensor rod 6 is eccentric to the sensor body 5a. In the case of the stroke sensor shown in FIG. 8, where the sensor rod 6 is eccentric to the sensor body 5a, the outside diameter of the sensor body 5a is reduced but it is necessary to select a large inside diameter of the sensor accommodation tube 7 as in the case of FIG. 7 because it is necessary that the center of the piston rod 2 and the center of the sensor rod 6 coincide with each other. Consequently, in either case of FIGS. 7 and 8, the arrangement is disadvantageous in that the stroke sensor accommodation space is increased so that the size of the fluid pressure cylinder is large and so that the manufacture cost is high. It is also disadvantageous in that the operating oil for the reciprocating motion of the piston rod 2 is spattered on the sensor at the time of inspection or replacement of the sensor body 5, and that since the cylinder bottom of the conventional type cylinders is of an integral type, the operation for inspection or replacement of the stroke sensor is laborious and entails a leak of the operating oil.
In consideration of these problems, it is an object of the present invention to provide a cylinder with a built-in stroke sensor which has a structure such that the stroke sensor accommodation space can be minimized, and whose stroke sensor can be inspected or replaced easily.